In recent years the extrapolation chamber has been developed a clinical dosimeter for electron beams and X-rays from medical linac. It consists of a new type extrapolation chamber, a water phantom and an intelligent portable instrument. The chamber is fitted a thin entrance window and has a collecting electrode made of polystyrene 20 mm in diameter. The electrode spacing can be varied by stepping motor drive achieving high precision of electrode setting. The variable gap is as small as 0.20 mm to 6 mm. The dosimeter can automatically finish the measuring process, and has error self-test and dose self-recording function. The total uncertainty is 2.7 %.